2005 Sequoia 2wd.....158k miles.......just had my rear differential rebuilt by M.I.T. Drivetrain Specialists in El Cajon, CA ( San Diego area ).
Problem: howling ( whirling noise ) gets louder with increasing speed and gets quieter with decreasing speed.
Diagnosis: differentials bearing rebuild ( 2 carrier bearings and the 2 bearings for the pinion gear. ). The ring, pinion gear, and spider gears themselves were fine.
Bearings wear out. This is more common than you think.
Anyway, rear differential is as quiet as the day it rolled off the assembly line. Only problem was that it was a good chunk of change to fix.
Yes, I am guilty of never changing fluid the differential fluid.